Digital camera

ABSTRACT

A digital camera includes: an image sensor that captures an image of a subject and outputs an image signal; and a control device that engages the image sensor to capture an image at an exposure value having been set, makes a decision as to whether or not an overflow or an underflow deviating from a dynamic range of the image sensor manifests, calculates an exposure correction quantity with which the exposure value having been set is to be corrected in order to reduce the overflow or the underflow based upon results of the decision and engages the image sensor to capture a new image at an exposure value having been corrected in correspondence to the exposure correction quantity.

The disclosures of the following priority application are hereinincorporated by reference:

Japanese Patent Application No. 2004-343539 filed Nov. 29, 2004

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a digital camera capable of producingimages less likely to include white areas or black areas attributable toover-exposure or under-exposure.

2. Description of Related Art

In a digital still camera, a subject light flux having been transmittedthrough a photographic lens is received at an image sensor such as aCCD, the received light flux undergoes photoelectric conversion, animage is obtained based upon the output resulting from the photoelectricconversion, and the image signal then undergoes various corrections andis finally recorded into an image recording medium such as a memorycard. The digital still camera may include a liquid crystal monitor atwhich images can be displayed, and in such a digital still camera, themonitor can be used as a viewfinder by sequentially updating the displayat the liquid crystal monitor with continuously captured images. Inaddition, the digital still camera may be capable of displaying at themonitor a graph indicating the brightness frequency distribution in animage, i.e., a histogram (see, for instance, Japanese Laid Open PatentPublication No. H7-38801).

SUMMARY OF THE INVENTION

Before capturing an image, exposure values (an aperture value and/or ashutter speed) is set in the camera so as to photograph the main subjectwith the optimal exposure. However, when photographing a scene in whichthere is a significant difference between the brightness of the mainsubject and the brightness of the background, the brightness in thebackground area may deviate from the dynamic range of the image sensor,resulting in the appearance of white areas due to over-exposure or blackareas due to under-exposure. While the appearance of such areas can bereduced by the photographer by performing exposure correction, thephotographer needs to have a great deal of experience to be able toselect the optimal exposure correction quantity. While the photographeris able to set the exposure correction quantity by checking thehistogram on a camera capable of providing real-time display of thehistogram, such an exposure correction operation is complicated andnovice users are likely to find it unsatisfactory.

According to the 1st aspect of the invention, a digital cameracomprises: an image sensor that captures an image of a subject andoutputs an image signal; and a control device that engages the imagesensor to capture an image at an exposure value having been set, makes adecision as to whether or not an overflow or an underflow deviating froma dynamic range of the image sensor manifests, calculates an exposurecorrection quantity with which the exposure value having been set is tobe corrected in order to reduce the overflow or the underflow based uponresults of the decision and engages the image sensor to capture a newimage at an exposure value having been corrected in correspondence tothe exposure correction quantity.

According to the 2nd aspect of the invention, in the digital cameraaccording to the 1st aspect, it is preferred that the control devicemakes a decision as to whether or not an overflow or an underflowmanifests and calculates the exposure correction quantity based upon theresults of the decision by using an image signal of an image capturedbefore an operation with an image-capturing button, and engages theimage sensor to capture a new image at the corrected exposure value inresponse to a shutter release operation.

According to the 3rd aspect of the invention, in the digital cameraaccording to the 2nd aspect, it is preferred that the control devicemakes a decision as to whether or not an overflow or an underflowmanifests and calculates the exposure correction quantity based upon theresults of the decision in response to a photographing preparation startoperation performed prior to the shutter release operation.

According to the 4th aspect of the invention, in the digital cameraaccording to the 3rd aspect, it is preferred that if the shutter releaseoperation is performed immediately following the photographingpreparation start operation, the control device engages the image sensorto capture an image at an initial exposure value having been set withoutmaking a decision as to whether or not an overflow or an underflowmanifests and calculating the exposure correction quantity based uponthe results of the decision.

According to the 5th aspect of the invention, in the digital cameraaccording to the 1st aspect, it is preferred that the control deviceascertains an overflow frequency count and an underflow frequency count,determines a direction for exposure correction based upon the overflowfrequency count and the underflow frequency count having beenascertained and determines the exposure correction quantity by takinginto consideration the direction.

According to the 6th aspect of the invention, in the digital cameraaccording to the 5th aspect, it is preferred that the control devicesets a plurality of exposure correction quantities as candidates incorrespondence to the direction for the exposure correction having beendetermined, corrects the exposure value having been set individually incorrespondence to each of the plurality of exposure correctionquantities having been set as the candidates, engages the image sensorto capture a plurality of images each at one of a plurality of exposurevalues resulting from correction, selects an optimal exposure correctionquantity among the candidates by analyzing a plurality of image signalseach corresponding to one of the plurality of images having beencaptured and engages the image sensor to capture a new image at anexposure value corrected in correspondence to the optimal exposurecorrection quantity.

According to the 7th aspect of the invention, in the digital cameraaccording to the 6th aspect, it is preferred that the control deviceengages the image sensor to capture the plurality of images insuccession, stores an image signal each time an image is captured, andanalyzes the plurality of image signals having been stored and selectsthe optimal exposure correction quantity after the plurality of imageshave been captured.

According to the 8th aspect of the invention, in the digital cameraaccording to the 1st aspect, it is preferred that the control deviceascertains a brightness frequency distribution in the image signal basedupon the image signal and makes a decision based upon the brightnessfrequency distribution having been ascertained as to whether or not anoverflow or an underflow deviating from the dynamic range of the imagesensor manifests.

According to the 9th aspect of the invention, in the digital cameraaccording to the 2nd aspect, it is preferred that the control devicerecords into a recording medium image data based upon a signal outputfrom the image sensor in response to the shutter release operation.

According to the 10th aspect of the invention, in the digital cameraaccording to the 2nd aspect, it is preferred that the shutter releaseoperation is performed by fully pressing down on a shutter releasebutton.

According to the 11th aspect of the invention, in the digital cameraaccording to the 3rd aspect, it is preferred that the photographingpreparation start operation is performed by pressing a shutter releasebutton halfway down.

According to the 12th aspect of the invention, in the digital cameraaccording to the 1st aspect, it is preferred that the image sensor isconstituted with a plurality of pixels and outputs a plurality ofsignals each corresponding to one of the pixels as the image signal.

According to the 13th aspect of the invention, in the digital cameraaccording to the 1st aspect, it is preferred that the overflow is asignal that causes white clipping in an image and the underflow is asignal that causes black clipping in an image.

According to the 14th aspect of the invention, a digital cameracomprises: an image sensor that is constituted with a plurality ofpixels, captures an image of a subject and outputs a plurality ofsignals each corresponding to one of the pixels; and a control devicethat controls an image-capturing operation at the image sensor. And thecontrol device controls the image sensor so as to capture a subjectimage at an exposure value having been set, counts a number of signalsindicating values exceeding a predetermined upper limit value and anumber of signals indicating values under a predetermined lower limitvalue among the plurality of signals output by capturing an image at theexposure value having been set, corrects the exposure value having beenset based upon the number of signals indicating values exceeding thepredetermined upper limit value and the number of signals indicatingvalues under the predetermined lower limit value having been counted andcontrols the image sensor so as to capture a subject image at thecorrected exposure value.

According to the 15th aspect of the invention, in the digital cameraaccording to the 14th aspect, it is preferred that the control devicecorrects the exposure value having been set toward an under-exposureside if signals with values exceeding the predetermined upper limitalone have been counted or the number of signals indicating valuesexceeding the predetermined upper limit value is greater than the numberof signals indicating values under the predetermined lower limit value,and corrects the exposure value having been set toward an over-exposureside if signals with values under the predetermined lower limit valuealone have been counted or the number of signals indicating values underthe predetermined lower limit value is greater than the number ofsignals indicating values exceeding the predetermined upper limit value.

According to the 16th aspect of the invention, in the digital cameraaccording to the 14th aspect, it is preferred that the control devicedetermines a direction for exposure correction with regard to theexposure value having been set based upon the number of signalsindicating values exceeding the predetermined upper limit value and thenumber of signals indicating values under the predetermined lower limitvalue having been counted, selects a plurality of correction valuesalong the direction for exposure correction having been determined,corrects the exposure value having been set individually by using eachof the plurality of correction values having been selected, controls theimage sensor so as to capture subject images each at one of a pluralityof corrected exposure values, determines an optimal corrected exposurevalue among the plurality of corrected exposure values by analyzing aplurality of signal sets each output as an image is captured at one ofthe plurality of corrected exposure values, and controls the imagesensor so as to capture a subject image at the optimal correctedexposure value having been determined.

According to the 17th aspect of the invention, in the digital cameraaccording to the 16th aspect, it is preferred that the control devicecounts signals indicating values exceeding the predetermined upper limitvalue and signals indicating values under the predetermined lower limitvalue among the plurality of signals output as an image is captured incorrespondence to each of the plurality of signal sets, and determinesthe optimal corrected exposure value corresponding to a signal set witha smallest number of signals indicating values exceeding thepredetermined upper limit value or a smallest number of signalsindicating values under the predetermined lower limit value.

According to the 18th aspect of the invention, in the digital cameraaccording to the 14th aspect, it is preferred that: there is furtherprovided a recording device that records into a recording medium imagedata based upon a plurality of signals output by the image sensor; andthe control device records into the recording medium image data basedupon a plurality of signals output by capturing a subject image at thecorrected exposure value without recording into the recording mediumimage data based upon a plurality of signals output as the image iscaptured at the exposure value having been set.

According to the 19th aspect of the invention, in the digital cameraaccording to the 16th aspect, it is preferred that: there is furtherprovided a recording device that records into a recording medium imagedata based upon a plurality of signals output by the image sensor; andthe control device records into the recording medium image data basedupon a plurality of signals output as a subject image is captured at theoptimal corrected exposure value having been determined withoutrecording into the recording medium image data based upon a plurality ofsignals output by capturing an image at the exposure value having beenset or a plurality of sets of image data corresponding to the pluralityof signal sets each output as an image is captured at one of theplurality of corrected exposure values.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a control block diagram related to the camera achieved in anembodiment of the present invention;

FIG. 2 presents a basic flowchart of camera operations;

FIG. 3 presents a detailed flowchart of the exposure value determinationprocessing, which includes automatic exposure correction control;

FIG. 4 presents a flowchart in continuation from FIG. 3;

FIG. 5 is a time chart indicating the timing with which individualoperations are executed when determining the exposure correctionquantity;

FIG. 6 is a time chart representing an example of the automatic exposurecorrection processing in which each image is analyzed as it is captured;and

FIG. 7 is a time chart representing another example of the automaticexposure correction processing, in which images are analyzed after allthe images are captured.

DESCRIPTION OF PREFERRED EMBODIMENT

In reference to FIGS. 1 to 7, an embodiment of the present invention isexplained.

FIG. 1 is a block diagram of the digital still camera according to thepresent invention. An image is formed at the light receiving surface ofan image sensor (an image-capturing element or device) 2 constitutedwith a plurality of pixels, such as a CCD, with a subject light fluxhaving been transmitted through a photographic lens 1. The image sensor2 captures the image and outputs an image signal (electrical signal)constituted with a plurality of pixel signals each corresponding to oneof the plurality of pixels and indicating the intensity of the light inthe subject image having been formed. The image signal, constituted withthe plurality of pixel signals, may be otherwise referred to as a signalset. The image signal is converted to a digital signal at an A/Dconverter 3, the digital signal then undergoes various types of imageprocessing at a signal processing circuit 4 and thus, image data aregenerated. The image data are temporarily stored into a buffer memory 5and are recorded into a memory card MC via a recording circuit 6. Theimage data may be compressed in a predetermined compression format at animage compression circuit 7 as necessary before they are recorded. Theindividual circuits described above are controlled by a CPU 8.

The photographic field is photometered by a photometering circuit 9 andthe results of the photometering operation are input to the CPU 8 beforean image is captured. The CPU 8 executes an exposure calculation basedupon the photometering results, the ISO sensitivity level and the likeand thus determines exposure values (an aperture value and a shutterspeed) that will allow the subject to be photographed with the optimalexposure. Once the exposure values are determined, the aperture (notshown) is adjusted based upon the aperture value and the image iscaptured over an exposure period the length of which corresponds to theshutter speed. At a liquid crystal monitor 10, which includes a liquidcrystal screen, a liquid crystal drive unit, a backlight, a backlightcontrol unit and the like, an image is displayed based upon image dataor other types of information are displayed at the screen thereof.

A halfway press switch SW1 that comes on in response to a halfway pressoperation (photographing preparation start operation) of a shutterrelease button, a shutter release switch SW2 that comes on in responseto a full press operation (a shutter release operation) of the shutterrelease button, other operation members 11 and a memory 12 constitutedwith a ROM, a RAM and the like for storing information needed forexecuting control are connected to the CPU 8. The full press operation(shutter release operation) of the shutter release button, whichconstitutes an instruction for an actual image-capturing (photographing)operation and also constitutes an instruction for recording the imagedata obtained through the image-capturing operation into the memory cardMC or the like, may be otherwise referred to as an image-capturinginstruction operation or an image data recording instruction operation.Likewise, the shutter release button may be referred to as animage-capturing instruction button or an image data recordinginstruction button.

As the power to the camera structured as described above is turned on,the photometering operation, the exposure calculation and theimage-capturing operation described above are repeatedly executed andimages each obtained through a single cycle of operations aresuccessively displayed at a liquid crystal monitor 10. The display modein which images successively captured are sequentially displayed,thereby constantly updating the display is referred to as a throughimage display mode. In this mode, the photographer is able to decide onthe image composition by checking the through images. It is to be notedthat through images are not recorded into the memory card MC.

The automatic exposure correction control, which is the feature thatcharacterizes the embodiment, is now explained. As the shutter releasebutton is pressed halfway down (SW1 ON) in the state described above,the CPU 8 determines an exposure correction quantity based upon whichthe extent of the appearance of white areas due to over-exposure orblack areas due to under-exposure is to be reduced while continuouslydisplaying through images. More specifically, it obtains a through imagebrightness (or luminance) frequency distribution (histogram) and, basedupon the distribution, it determines whether or not an overflow or anunderflow manifests, i.e., whether or not there are any pixel signalsdeviating from the dynamic range of the image sensor 2 toward theover-exposure side or the under-exposure side. Since an overflow willresult in the appearance of white areas (white-clipping or whitesaturation) and an underflow will result in the appearance of blackareas (black-clipping or black saturation), the exposure will need to becorrected toward the under-exposure side in order to reduce theappearance of white areas if an overflow manifests, whereas the exposurewill need to be corrected toward the over-exposure side in order toreduce the appearance of black areas if an underflow manifests.

Accordingly, if an overflow or an underflow is detected, an image iscaptured a plurality of times by correcting the initial exposure valueto varying extents, brightness frequency distributions are obtained eachbased upon a given set of image-capturing results (each image) and theexposure correction quantity having been set for the image with theleast overflow or underflow frequency count is extracted in theembodiment. If there is a plurality of images with the least overflowfrequency count, the exposure correction quantity with the smallestabsolute value is extracted, so as to stay as close as possible to theinitial exposure value while reducing the extent of the appearance ofwhite areas due to over-exposure or black areas due to under-exposure.The initial exposure value is corrected based upon the exposurecorrection quantity thus extracted and the exposure value is reset.Subsequently, in response to a shutter release operation (SW2 ON), animage-capturing operation is executed at the adjusted exposure value andthe resulting image data are recorded into the memory card MC.

Through the automatic exposure correction control described above, animage manifesting a lower extent of appearance of white areas due toover-exposure or black areas due to under-exposure is ultimatelyrecorded. In addition, since the camera automatically decides whether toexecute white area due to over-exposure reduction processing or theblack area due to under-exposure reduction processing and obtains theoptimal exposure correction quantity for either the white area due toover-exposure reduction or black area due to under-exposure reduction,even a novice photographer is likely to be able to produce an optimallyexposed photograph simply by performing a photographing operationwithout having to concern himself with exposure. Furthermore, since theprocessing for determining the exposure correction quantity is executedduring the halfway press operation, a time lag to elapse between theshutter release operation and the image recording operation is exactlythe same as that elapsing during a regular photographing operation,thereby assuring stress-free handling of the camera for thephotographer.

As an alternative, a plurality of images may be captured at varyingexposure values in response to a shutter release operation and an imagemanifesting the least extent of appearance of white areas due toover-exposure or black areas due to under-exposure among the pluralityof images may be selected and recorded. However, this alternative is notideal since the time lag to elapse between the shutter release operationand the recording completion is bound to be protracted. In this case,since the photographer cannot take the next picture immediately andthus, he may miss a good photo opportunity.

In reference to FIGS. 2 to 4, each showing a processing procedure forexecuting the control described above in software, the automaticexposure correction operations are explained in further detail.

FIG. 2 shows the basic camera operations executed based upon a program.As the power to the camera is turned on, the program is started up bythe CPU 8, and in step S001, power ON processing is executed. As thepower ON processing is executed, through-image-capturing, executed over1/30 sec time intervals, starts. In step S002, the liquid crystalmonitor 10 is turned on and in step S003, through image display at theliquid crystal monitor 10 starts.

In step S101, AE processing, constituted with the photometeringoperation and the exposure calculation explained earlier, is executedand the exposure values are determined. In step S102, AF processing isexecuted and the photographic lens 1 is driven to the focus matchposition based upon the results of focal point detection executed by arange finding device (not shown). The processing in steps S101 and S102is executed repeatedly until it is decided that a halfway pressoperation has been performed in step S103 or until power OFF isindicated in step S104. Each set of the AF/AE results is reflected inthe corresponding through image on the through image display having beeninitially started in step S003. If it is verified that power OFF hasbeen indicated, the liquid crystal monitor 10 is turned off in step S201and power OFF processing is executed in step S202 before the processingsequence ends.

If, on the other hand, it is decided in step S103 that a halfway pressoperation has been performed (SW1 ON), an AF lock is set in step S301 tohold the photographic lens in the focus matched position, and then,exposure determination processing is executed in step S302. The exposuredetermination processing executed at this time, which includes theautomatic setting processing for automatically setting the exposurecorrection quantity explained earlier, is to be described in detaillater.

Following step S302, the operation follows through the loop made up withsteps S401 and 402 to wait for a shutter release operation or a halfwaypress operation clearance. If it is decided that the halfway pressoperation has been cleared (SW1 OFF), the operation returns to step S101and the processing described above is repeatedly executed. If, on theother hand, it is decided that a shutter release operation has beenperformed (SW2 ON), photographing processing is executed in step S501.This photographing processing includes aperture control executed basedupon the aperture value having been determined in step S302 and animage-capturing operation executed based upon the shutter speed havingbeen determined in step S302. In step S502, the through image display iscleared, and instead, the image having been obtained in step S501 isbrought up on display at the liquid crystal monitor 10. Then, in stepS503, the image data are recorded into the memory card MC.

In step S504, the operation waits for the shutter release operation tobe cleared and, once the shutter release operation is cleared, it waitsfor the halfway press operation to be cleared in step S505. Once thehalfway press operation is cleared, the monitor display is reset to thethrough image display mode in step S506 before the operation returns tostep S101.

Next, the exposure determination processing executed in step S302 isexplained in detail in reference to FIGS. 3 and 4.

In step S1001, the photometering operation and the exposure calculationare executed and the aperture value and the shutter speed are thusdetermined through the arithmetic operation. These values are stored asthe exposure value settings (the aperture value setting and the shutterspeed setting). In step S1002, the aperture is controlled based upon theaperture value setting. In this state, an image is captured at theshutter speed having been set, and the resulting image is displayed as athrough image.

In step S1002A, a decision is made as to whether or not the camera iscurrently set in an automatic exposure correction mode. If it is decidedthat the camera is not currently set in the automatic exposurecorrection mode, which can be selected/cleared via a specific operationmember or via a menu setting screen, the operation proceeds to stepS1401 in FIG. 4 to effect an AE lock. In this case, the automaticexposure correction is not executed and the exposure value settings (thevalues calculated in step S1001) are fixed as the final exposure values.If, on the other hand, it is decided that the camera is currently set inthe automatic exposure correction mode, a decision is made in step S1003as to whether or not a shutter release operation has been performed (SW2ON/OFF), and the operation proceeds to step S1004 to ascertain thebrightness frequency distribution in the through image currently ondisplay if it is decided that no shutter release operation has beenperformed. Then, based upon the brightness frequency distribution, anoverflow frequency count and an underflow frequency count aredetermined.

In steps S1005A through S1005D, overflow/underflow decision-makingprocessing is executed based upon the brightness frequency distributionascertained in step S1004. If it is decided in step S1005A that neitheran overflow nor an underflow has manifested the operation proceeds tostep S1401 to effect an AE lock, since there is no need for exposurecorrection. In this case, too, the exposure value settings are fixed asthe final exposure value.

If an overflow alone is detected, a white area reduction exposuresetting mode is selected in step S1101, whereas if an underflow alone isdetected, a black area reduction exposure setting mode is selected instep S1201. If both an overflow and an underflow are detected, theirfrequency counts are compared. The operation proceeds to step S1101 ifthe overflow count is equal to or greater than the underflow count, butthe operation proceeds to step S1201 otherwise.

Namely, through the processing executed in steps S1005A through S1005D,the direction of the correction is determined, i.e., a decision is madewhether to correct the exposure toward the over-exposure side or towardthe under-exposure side.

In the processing, if an overflow and an underflow manifest tosubstantially equal extents, the correction priority is given to thereduction of white areas due to over-exposure, since human visualperception characteristics are such that we tend to find white areas dueto over-exposure more visually disturbing than black areas due tounder-exposure. For this reason, even when the underflow frequency countis greater than the overflow frequency count, the operation may proceedto execute the white area reduction exposure setting mode processing ifthe overflow frequency count is equal to or greater than a predeterminedvalue. In addition, overflow/underflow manifesting to slight extents maybe regarded as no overflow/underflow if they are within allowableranges.

Once the white area reduction exposure setting mode is set, exposurecorrection quantities A through E are set in step S1102. The exposurecorrection quantities A through E are set in stages relative to theexposure value setting so that A=+0.5:B=0:C=−0.5:D=−1.0:E=−1.5. Sincethe object is to reduce the appearance of white areas due toover-exposure, the correction range is mainly set over theunder-exposure side. In this case, the exposure correction quantities C,D and E alone may be set. If, on the other hand, the black areareduction exposure setting mode is selected, exposure correctionquantities A through E are set in a similar manner in step S1202. Theexposure correction quantities A through E are set so that A=+1.5:B=+1.0:C=+0.5:D=0:E=−0.5 in this mode. Since the object is toreduce the appearance of black areas due to under-exposure, thecorrection range is mainly set over the over-exposure side. In thiscase, the exposure correction quantities A, B and C alone may be set.

Subsequently, through the processing executed in steps S1301 throughS1316, the exposure correction values A through E are sequentiallyselected, an image is captured at the exposure value corrected incorrespondence to each selected correction quantity and the resultingimage is displayed as a through image. The “V synchronization (verticalsynchronization)” indicates the length of time (= 1/30 sec) of timeintervals over which the through images are captured. The exposurecorrection is executed by adjusting the shutter speed in correspondenceto the values A through E while the aperture value remains fixed at theaperture value setting. It is to be noted that the exposure correctionmay instead be executed by adjusting the aperture value.

FIG. 5 is a time chart of the operations executed through the processingin steps S1301 through S1316. As the first exposure correction quantityA is set, a through image is exposed (captured) at the exposure valuecorrected based upon the exposure correction quantity A with thesubsequent V synchronous timing, and in synchronization with thisthrough image exposure, the next exposure correction quantity B is set.With the next V synchronous timing, the image obtained through thethrough image exposure (corresponding to the correction quantity A) isread, and in synchronization with the image read, a through image isexposed (in correspondence to the correction quantity B) and theexposure correction quantity C is set. Then, with the next V synchronoustiming, the through image having been read is displayed (correspondingto the correction quantity A), the image (corresponding to thecorrection quantity B) is read, a through image (corresponding to thecorrection quantity C) is exposed and the exposure correction quantity Dis set. Thus, the operations are executed with the V synchronous timinguntil the last through image (corresponding to the correction quantityE) is displayed, and the brightness frequency distributions areextracted in correspondence to the individual through images (incorrespondence to the individual correction quantities A through E).

In step S1317, the five brightness frequency distributions are analyzedand the optimal exposure correction quantity is determined. Namely, ifthe white area reduction exposure setting mode is currently set, theexposure correction quantity corresponding to the image with the leastoverflow frequency count among the exposure correction quantities Athrough E is extracted. If, on the other hand, the black area reductionexposure setting mode is currently set, the exposure correction quantitycorresponding to the image with the least underflow frequency countamong the exposure correction quantities A through E is extracted. Ifthere is a plurality of images with the least overflow/underflowfrequency count, the exposure correction value with the smallestabsolute value should be extracted so as to capture an image at anexposure value closer to the exposure value setting. The exposurecorrection quantity thus extracted is the optimal exposure correctionquantity and the exposure value corrected based upon the optimalexposure correction quantity is the final exposure value. In step S1318,a through image captured at the final exposure value is displayed, andin step S1401, the final exposure value is held through the AE lockbefore the operation makes a return.

The processing described above is executed after a negative decision ismade in step S1003. If an affirmative decision is made in step S1003,the operation directly proceeds to step S1401. An affirmative decisionmade in step S1003 indicates that a shutter release operation has beenperformed in immediate succession to a halfway press operation, i.e.,the halfway press operation has directly shifted into a full pressoperation without holding the halfway press state. In such a case, it isjudged that the photographer wishes to take a photograph immediatelyand, accordingly, the operation skips the processing in step S1004 andsubsequent steps even if the automatic exposure correction mode iscurrently set. Instead, an AE lock is set to fix the exposure valuesettings as the final exposure values in preparation for a photographingoperation. As a result, the likelihood of catching a good photoopportunity is increased.

FIGS. 6 and 7 are time charts of two different methods of automaticexposure correction processing.

In the method shown in FIG. 6, the brightness frequency distribution isanalyzed each time an image is captured, and the individual operations,i.e., photographing (image-capturing), image read, image processing,display and analysis, are each executed with V synchronous timing. Thenumerals attached to the letter V each indicate the number of operationshaving been executed at the corresponding time point. In this method,the next image is captured after the preceding image has been analyzed,and thus, 20V (V= 1/30sec)=667 ms elapses before analyzing of the fifthimage is completed, i.e. before the optimal exposure correction value isdetermined. However, since images having already been captured do notneed to be saved, the buffer memory does not need to have a largecapacity.

Higher-speed processing is achieved through the method shown in FIG. 7,by capturing images successively, storing the individual images as theyare captured and obtaining and analyzing their brightness frequencydistributions after all the images are stored. In this case, the buffermemory needs to have a large enough capacity to save at least fiveimages. However, since each image can be captured without having to waitfor the analysis of the preceding image to be completed, theoptimal-exposure correction value can be determined in 8V=267 ms. Thus,the automatic exposure correction control is executed too fast to benoticed by the photographer.

It is to be noted that while five images are captured to determine theoptimal exposure correction quantity in the example explained above, theimage-capturing operation does not need to be executed five times. Inaddition, the processing may be simplified by skipping theimage-capturing operations and univocally determining the exposurecorrection quantity in correspondence to the overflow (or underflow)frequency count. For instance, if the overflow (or underflow) frequencycount is less than a predetermined value, the exposure value setting maybe used without correction, the exposure value setting may be correctedby −1 stage (or +1 stage) if the frequency count is equal to or greaterthan a predetermined value within a specific range, and the exposurevalue setting may be corrected by −2 stage (or +2 stage) if thefrequency count is beyond the range. Through this method, too, a certainextent of correction effect is achieved. Furthermore, it is desirable toset certain limits with regard to the optimal exposure correctionquantity. If the exposure value setting is corrected by an excessiveextent, the areas to be correctly exposed (in particular the mainsubject) may become greatly under-exposed or greatly over-exposed whilethe appearance of white areas due to over-exposure or black areas due tounder-exposure is reduced.

In addition, while the exposure correction quantity is selected inresponse to a halfway press operation, they may be obtained in responseto a shutter release operation instead. While the shutter release timelag may become protracted in the latter case, the appearance of whiteareas due to over-exposure or black areas due to under-exposure can beeffectively eliminated without the photographer being particularly awareof the process.

An explanation is given above in reference to the embodiment on anexample in which the brightness frequency distribution is ascertainedand a decision is made as to whether or not an overflow or an underflowmanifests, i.e., whether or not there are pixel signals deviating fromthe dynamic range of the image sensor 2 toward the over-exposure side orthe under-exposure side. However, it is not strictly necessary toascertain the brightness frequency distribution. A decision as towhether or not an overflow or an underflow manifests may be made simplyby judging whether or not the image signal corresponding to each pixelindicates a value equal to or greater than a predetermined over-exposureside threshold value (an upper limit value) or judging whether or notthe value is equal to or less than a predetermined under-exposure sidethreshold value (a lower limit value) Then, the overflow frequency countcan be determined by counting the image signals with values equal to orgreater than the predetermined over-exposure side threshold value (upperlimit value) and the underflow frequency count can be determined bycounting image signals with values equal to or less than thepredetermined under-exposure side threshold value (lower limit value).

While an explanation is given above in reference to the embodiment on anexample in which the present invention is adopted in a digital stillcamera, the present invention is not limited to this example. Thepresent invention may instead be adopted in conjunction with imagescaptured on a camera mounted at a portable telephone. In addition, thepresent invention may be adopted in conjunction with still imagesphotographed with a video camera or the like. Namely, the presentinvention may be adopted in all types of apparatuses in which a subjectimage is captured at an image sensor by setting a specific exposurevalue. Such cameras and apparatuses that invariably handle digital imagesignals may be regarded as digital cameras.

The above described embodiment is an example and various modificationscan be made without departing from the spirit and scope of theinvention.

1. A digital camera comprising: an image sensor that captures an imageof a subject and outputs an image signal; and a control device thatengages the image sensor to capture an image at an exposure value havingbeen set, makes a decision as to whether or not an overflow or anunderflow deviating from a dynamic range of the image sensor manifests,calculates an exposure correction quantity with which the exposure valuehaving been set is to be corrected in order to reduce the overflow orthe underflow based upon results of the decision and engages the imagesensor to capture a new image at an exposure value having been correctedin correspondence to the exposure correction quantity.
 2. A digitalcamera according to claim 1, wherein: the control device makes adecision as to whether or not an overflow or an underflow manifests andcalculates the exposure correction quantity based upon the results ofthe decision by using an image signal of an image captured before anoperation with an image-capturing button, and engages the image sensorto capture a new image at the corrected exposure value in response to ashutter release operation.
 3. A digital camera according to claim 2,wherein: the control device makes a decision as to whether or not anoverflow or an underflow manifests and calculates the exposurecorrection quantity based upon the results of the decision in responseto a photographing preparation start operation performed prior to theshutter release operation.
 4. A digital camera according to claim 3,wherein: if the shutter release operation is performed immediatelyfollowing the photographing preparation start operation, the controldevice engages the image sensor to capture an image at an initialexposure value having been set without making a decision as to whetheror not an overflow or an underflow manifests and calculating theexposure correction quantity based upon the results of the decision. 5.A digital camera according to claim 1, wherein: the control deviceascertains an overflow frequency count and an underflow frequency count,determines a direction for exposure correction based upon the overflowfrequency count and the underflow frequency count having beenascertained and determines the exposure correction quantity by takinginto consideration the direction.
 6. A digital camera according to claim5, wherein: the control device sets a plurality of exposure correctionquantities as candidates in correspondence to the direction for theexposure correction having been determined, corrects the exposure valuehaving been set individually in correspondence to each of the pluralityof exposure correction quantities having been set as the candidates,engages the image sensor to capture a plurality of images each at one ofa plurality of exposure values resulting from correction, selects anoptimal exposure correction quantity among the candidates by analyzing aplurality of image signals each corresponding to one of the plurality ofimages having been captured and engages the image sensor to capture anew image at an exposure value corrected in correspondence to theoptimal exposure correction quantity.
 7. A digital camera according toclaim 6, wherein: the control device engages the image sensor to capturethe plurality of images in succession, stores an image signal each timean image is captured, and analyzes the plurality of image signals havingbeen stored and selects the optimal exposure correction quantity afterthe plurality of images have been captured.
 8. A digital cameraaccording to claim 1, wherein: the control device ascertains abrightness frequency distribution in the image signal based upon theimage signal and makes a decision based upon the brightness frequencydistribution having been ascertained as to whether or not an overflow oran underflow deviating from the dynamic range of the image sensormanifests.
 9. A digital camera according to claim 2, wherein: thecontrol device records into a recording medium image data based upon asignal output from the image sensor in response to the shutter releaseoperation.
 10. A digital camera according to claim 2, wherein: theshutter release operation is performed by fully pressing down on ashutter release button.
 11. A digital camera according to claim 3,wherein: the photographing preparation start operation is performed bypressing a shutter release button halfway down.
 12. A digital cameraaccording to claim 1, wherein: the image sensor is constituted with aplurality of pixels and outputs a plurality of signals eachcorresponding to one of the pixels as the image signal.
 13. A digitalcamera according to claim 1, wherein: the overflow is a signal thatcauses white clipping in an image and the underflow is a signal thatcauses black clipping in an image.
 14. A digital camera comprising: animage sensor that is constituted with a plurality of pixels, captures animage of a subject and outputs a plurality of signals each correspondingto one of the pixels; and a control device that controls animage-capturing operation at the image sensor, wherein: the controldevice controls the image sensor so as to capture a subject image at anexposure value having been set, counts a number of signals indicatingvalues exceeding a predetermined upper limit value and a number ofsignals indicating values under a predetermined lower limit value amongthe plurality of signals output by capturing an image at the exposurevalue having been set, corrects the exposure value having been set basedupon the number of signals indicating values exceeding the predeterminedupper limit value and the number of signals indicating values under thepredetermined lower limit value having been counted and controls theimage sensor so as to capture a subject image at the corrected exposurevalue.
 15. A digital camera according to claim 14, wherein: the controldevice corrects the exposure value having been set toward anunder-exposure side if signals with values exceeding the predeterminedupper limit alone have been counted or the number of signals indicatingvalues exceeding the predetermined upper limit value is greater than thenumber of signals indicating values under the predetermined lower limitvalue, and corrects the exposure value having been set toward anover-exposure side if signals with values under the predetermined lowerlimit value alone have been counted or the number of signals indicatingvalues under the predetermined lower limit value is greater than thenumber of signals indicating values exceeding the predetermined upperlimit value.
 16. A digital camera according to claim 14, wherein: thecontrol device determines a direction for exposure correction withregard to the exposure value having been set based upon the number ofsignals indicating values exceeding the predetermined upper limit valueand the number of signals indicating values under the predeterminedlower limit value having been counted, selects a plurality of correctionvalues along the direction for exposure correction having beendetermined, corrects the exposure value having been set individually byusing each of the plurality of correction values having been selected,controls the image sensor so as to capture subject images each at one ofa plurality of corrected exposure values, determines an optimalcorrected exposure value among the plurality of corrected exposurevalues by analyzing a plurality of signal sets each output as an imageis captured at one of the plurality of corrected exposure values, andcontrols the image sensor so as to capture a subject image at theoptimal corrected exposure value having been determined.
 17. A digitalcamera according to claim 16, wherein: the control device counts signalsindicating values exceeding the predetermined upper limit value andsignals indicating values under the predetermined lower limit valueamong the plurality of signals output as an image is captured incorrespondence to each of the plurality of signal sets, and determinesthe optimal corrected exposure value corresponding to a signal set witha smallest number of signals indicating values exceeding thepredetermined upper limit value or a smallest number of signalsindicating values under the predetermined lower limit value.
 18. Adigital camera according to claim 14, further comprising: a recordingdevice that records into a recording medium image data based upon aplurality of signals output by the image sensor, wherein: the controldevice records into the recording medium image data based upon aplurality of signals output by capturing a subject image at thecorrected exposure value without recording into the recording mediumimage data based upon a plurality of signals output as the image iscaptured at the exposure value having been set.
 19. A digital cameraaccording to claim 16, further comprising: a recording device thatrecords into a recording medium image data based upon a plurality ofsignals output by the image sensor, wherein: the control device recordsinto the recording medium image data based upon a plurality of signalsoutput as a subject image is captured at the optimal corrected exposurevalue having been determined without recording into the recording mediumimage data based upon a plurality of signals output by capturing animage at the exposure value having been set or a plurality of sets ofimage data corresponding to the plurality of signal sets each output asan image is captured at one of the plurality of corrected exposurevalues.